SK Telecom, which controls more than half of the country's mobile phone market, asked the Korea Communications Commission to probe marketing activities by KT Corp and LG Uplus, to which it lost a net 23,800 customers from the beginning of May to June 14.

"(We) need prompt and selective measures aimed at operators promoting excessive competition in a bid to achieve the government's goal of curtailing marketing costs and boosting investment," SK Telecom said in a statement.

The complaint comes after SK Telecom announced a mobile tariff savings plan worth $696 million annually earlier this month as it came under pressure from the government trying to tame high inflation.

It said KT was spending as much as $684 in subsidies per customer to lure new smartphone subscribers.

KT had no immediate comment and officials at LG Uplus were not available for comment.

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